# Swirl Inlet

Source: https://help.sim-flow.com/boundary-conditions/swirl-inlet
Last updated: 2026-10-01

## Description

Swirl Inlet is a "Fixed Value" type velocity boundary condition that describes a swirling inflow, by specifying a central axis, central point, axial, radial, and tangential velocity profiles. The three components of swirl flow (axial, radial, and tangential) can be constant or time-dependent functions.

## Understanding Swirl Inlet

Mathematically, Swirl Inlet constructs the inlet velocity vector in the following way:

\[\vec U = U_z + U_r + U_{\phi}\]

where:

- \(U_z\) - axial component
- \(U_r\) - radial component
- \(U_{\phi}\) - tangential component

Each component is calculated in the following way:

\[U_z = U_{axial} \cdot \vec a \\ U_r = U_{radial} \cdot \vec r \\ U_\phi = U_{tangential} (\vec a \otimes \vec r)\]

where:

- \(\vec a\) - unit-length vector in an axial direction
- \(\vec r\) - unit-length vector in a radial direction

Axial (\(U_{axial}\)), radial (\(U_{radial}\)) and tangential velocity (\(U_{tangential}\)) components are taken directly from the User input.

Swirl Inlet is similar to [Cylindrical Inlet](https://help.sim-flow.com/boundary-conditions/cylindrical-inlet). However, the key difference is the definition of tangential velocity components. The first one (Swirl Inlet) allows the User to define a real velocity profile, while the latter defines revolutions per minute which are later internally transformed to rotational speed \(\omega\). Additionally, when a Table option is used (see Swirl Inlet in SimFlow section), all velocity components are radially dependent, and not time-dependent like for [Cylindrical Inlet](https://help.sim-flow.com/boundary-conditions/cylindrical-inlet).

## Application & Physical Interpretation

Swirl Inlet describes an inflow naturally expressed in cylindrical coordinates. It is perfectly suited for CFD simulations of cyclone separators, swirl burners, annular sonic nozzles or rotating-mesh mixers.

### Swirl Inlet in Combustor applications

**Example applications**: swirl-stabilized combustor

This problem can be addressed using the [sprayFoam](https://help.sim-flow.com/solvers/spray-foam) solver. While fully developed laminar flow in a pipe has a parabolic velocity profile, near the entrance, the profile is developing and not yet parabolic.

Example Boundary Conditions set for Combustor applications

| Physics | Pressure | Velocity |
| --- | --- | --- |
| Inlet | Zero Gradient | **Swirl Inlet** |
| Outlet (wind tunnel) | Fixed Value | Pressure Inlet-Outlet Velocity |

## Swirl Inlet in SimFlow

To define Swirl Inlet on the domain’s inlet, the proper option must be selected from the drop-down menu for velocity in the Boundary Conditions tab - [Figure 1](https://help.sim-flow.com/boundary-conditions/swirl-inlet#fig-sketch-01).

![Swirl Inlet boundary condition in SimFlow](https://help.sim-flow.com/boundary-conditions/images/swirl-inlet/swirl-inlet-bc-01.jpg)

Figure 1. Swirl Inlet boundary condition in SimFlow

The following parameters need to be defined:
Axis - axis of the coordinate system,
Origin - origin of the coordinate system,
Axial - axial velocity,
Radial - radial velocity,
Tangential - rotational speed used to define the tangential velocity component.

Axial, radial and RPM can be defined as constant values or as time-dependent values, which is presented in [Figure 2](https://help.sim-flow.com/boundary-conditions/swirl-inlet#fig-sketch-02) and [Figure 3](https://help.sim-flow.com/boundary-conditions/swirl-inlet#fig-sketch-03).

![Swirl Inlet boundary condition in SimFlow - Table](https://help.sim-flow.com/boundary-conditions/images/swirl-inlet/swirl-inlet-bc-02.jpg)

Figure 2. Swirl Inlet boundary condition in SimFlow - Table

When the table definition is used, the radial dependency is introduced.

![Swirl Inlet boundary condition in SimFlow](https://help.sim-flow.com/boundary-conditions/images/swirl-inlet/swirl-inlet-bc-03.jpg)

Figure 3. Swirl Inlet boundary condition in SimFlow - Table Input

## Swirl Inlet - Alternatives

In this section, we propose boundary conditions that are alternative to Swirl Inlet. While they may fulfill similar purposes, they might be better suited for a specific application and provide a better approximation of physical world conditions.

| Boundary Condition | Description |
| --- | --- |
| [Fixed Value](https://help.sim-flow.com/boundary-conditions/fixed-value) | specifies a constant value on the patch |
| [Cylindrical Inlet](https://help.sim-flow.com/boundary-conditions/cylindrical-inlet) | specifies axial and radial velocity in cylindrical systems |
| [Swirl Flow Rate Inlet](https://help.sim-flow.com/boundary-conditions/swirl-flow-rate-inlet) | specifies an inlet flow rate with a swirling component and defines the velocity field based on the desired flow rate |